Hydraulic lash adjusters are well known for use in internal combustion engines. Hydraulic lash adjuster mechanisms are used to eliminate clearance or lash between engine valve train components which can occur under varying operating conditions. Hydraulic lash adjusters are also used in order to maintain engine efficiency, reduce engine noise, and reduce wear in the valve train.
Hydraulic lash adjusters operate by transmitting the energy of the valve actuating cam through oil trapped in a pressure chamber beneath a plunger. During each operation of the cam, as the length of the valve actuating components varies due to temperature changes for example, small quantities of hydraulic fluid are permitted to enter or escape from the pressure chamber. As the hydraulic fluid enters or escapes the pressure chamber, the position of the plunger is adjusted and consequently the effective total length of the valve train is adjusted which minimizes or eliminates the lash.
Conventional hydraulic lash adjusters have a leak down rate controlled by a leak path defined by precise clearance between two concentric tubes, namely, the plunger and the outer cylinder, such as disclosed in U.S. Pat. No. 5,622,147. The leak down rate must be sufficiently fast so that as the exhaust valve heats and expands, the lash adjuster can relax and accommodate the expansion. If the leak down rate is too slow, the exhaust valve may not seat completely, potentially causing engine problems such as loss of power output and deposit buildup on the valve stem. These problems can be exacerbated with engine strategies that deliberately create high-temperature exhaust to quickly light off the catalyst, with the result that the exhaust valve also quickly heats and expands. While lash adjusters can quickly compensate for component shrinkage, they require more time to compensate for component expansion.
Similarly, a lash adjuster leak clown rate that is too fast can cause the adjuster to relax sufficiently during a single cycle that the cam follower loses contact with the cam. When this occurs, the exhaust valve can slam shut, causing noise which is most evident under hot idle conditions. Furthermore, since the leak down rate varies with oil viscosity, both the grade of oil used and the temperature will affect the leak down rate. Thus, current lash adjusters with fixed leak paths may be unable to provide leak down rates that are satisfactory for all operating conditions. For example, when an engine's oil is cold, and thus highly viscous, the leak down rate is slow. Because the leak path between the plunger and the lash adjuster body remains constant prior adjusters cannot increase the leak down to compensate for this condition.
Moreover, since the leak down rate depends strongly on the magnitude of the gap between the two concentric tubes, slight changes in dimensions will have a large effect on the leak down rate. As a result, these tubes typically are provided with a lapped finish and are matched to provide the required accuracy in leak down rate. Providing lash adjusters with consistent leak paths is an expensive process.